Bonding electrochemical cell components

ABSTRACT

The invention provides a method for preparing subassemblies for an electrochemical cell or a stack of electrochemical cells, particularly a stack of fuel cells for the direct generation of electricity. The method includes bonding together two or more electrochemical cell components, such as plates, frames, flow fields, shims, gaskets, membranes and the like, to form subassemblies used to make an electrochemical cell stack. The bonding can be accomplished using either polymeric bonds (i.e., adhesives) where polymer and/or metal components are involved or metallurgical bonds (i.e., solder) where metal components are involved. The bonding provides tightly sealed cells and lower electronic contact resistances between components.

FIELD OF THE INVENTION

The present invention relates to electrochemical cells and methods forassembling electrochemical cells.

BACKGROUND OF THE INVENTION

Electrochemical cells utilizing a proton exchange membrane (PEM) can beconfigured in cells stacks having bipolar separator plates betweenadjacent cells. These bipolar separator plates are typically made from avariety of metals, such as titanium and stainless steel, andnon-metallic conductors, such as graphitic carbon. Bipolar separatorplates are typically fabricated by machining flow fields into a solidsheet of the material. Alternatively, when carbonaceous conductivematerials are used, the precursor material is formed by injectionmolding and converted to the conductive carbon form by high temperaturefiring under carefully controlled conditions. The flow fields are madeup of a series of channels or grooves that allow passage of gases andliquids.

FIG. 1 is a face view of a prior art bipolar separator plate 10 madefrom a solid sheet of a conducting material. The central portion of theplate has a flow field 12 machined into its surface. The flow field maydirect fluid flow across the surface of an electrode in many patterns,but is illustrated here as parallel serpentine channels. Around theperimeter of the flow field 12, the plate provides a plurality of boltholes 14 for assembling and securing a cell stack, various manifolds 16for communicating fluids through the stack, and a flat surface 18 thatallows the plate to be sealed against adjacent components of the cellstack.

In addition to providing a fluid flowfield, a bipolar separator platefor use in electrochemical cells must collect electrons liberated at oneelectrode (i.e., an anode), conduct the electrons through the plate, anddeliver electrons to the face of another electrode (i.e., a cathode) onthe opposing side of the plate. The bipolar plate shown in FIG. 1,collects and delivers electrons from electrodes of opposing cellsthrough contact between the electrodes and the ridges 20 remainingbetween the channels 22 in the flowfield 12.

FIG. 2 is a schematic view of a proton exchange membrane (PEM)electrochemical cell configured as a hydrogen-air fuel cell stack 30.This stack 30 comprises two identical fuel cells 32 each having acathode 34, a PEM 36 and an anode 38. Flow fields 40 (shownschematically for clarity) are provided on either side of the bipolarseparator plate 42, as well as on the internal faces of the endplates44. Electrons liberated at the anodes 38 induce electronic current flowto the cathode 34 of an adjacent cell on the other side of the plate 42and, in the case of the last anode of the stack (here the anode on theright of the page), through an external circuit 46. Electrons are thencombined with protons and oxygen at the cathodes 34 to form water. Theelectrical potential of the fuel cell 30 is increased by adding morecells 32 to the stack.

While the foregoing methods are relatively straight forward, they haveseveral disadvantages. First, the solid piece of graphite or metal usedto fabricate the bipolar separator plate constrains the density of thefinal product to a density approximately the same as that of theoriginal stock, thereby producing a very dense and heavy bipolarseparator plate. Second, machining each piece from a solid startingblank requires relatively expensive machining processes, as opposed toless expensive molding, casting or stamping processes. When carboncomponents are used the molding step is inexpensive, however, thecontrolled sintering required to convert the precursor to the finalproduct is slow and requires precise atmosphere and temperature controlthroughout the process.

Another important aspect in fabricating an electrochemical fuel cell isthe number of joints and junctions created in the cell. Reduction of thenumber of joints and junctions can greatly improve the performance of aelectrochemical cell stack, for example if fabricated from a stack offlat components, because there are fewer potential leak points and fewerelectronic contact resistances. A fabrication process that provides anelectrochemical cell with a minimum of joints and junctions would behighly desirable.

Assembling a PEM fuel cell stack using relatively flat componentsrequires gas tight seals at each interface. Gaskets are typically usedto create gas tight seals, however gaskets increase the number of partsthat must be fabricated and aligned when the stack is assembled. Amethod and apparatus for forming gas tight bonds or seals at theinterfaces between the components of an electrochemical cell wouldobviate the need for several gaskets and produce a more efficient cell.

Therefore, there remains a need for an improved bipolar separator plate.It would be desirable if the bipolar separator plate were thin, lightweight, and could support high current densities. It would be furtherdesirable if the bipolar separator plate reduced the number of joints orjunctions in the individual cells or a cell stack and reduced the needfor gaskets. Furthermore, it would be useful if the structure of thebipolar separator plate allowed the introduction of other specificproperties, such as water permeability and reactant gas impermeability.

SUMMARY OF THE INVENTION

The present invention provides a method for preparing a subassembly foran electrochemical cell. The method includes aligning a subassemblyhaving two or more electrochemical cell components with one or morebonding elements disposed between the two or more electrochemical cellcomponents. The bonding elements have a melting point temperature thatis lower than the melting point temperature of any one of the two ormore electrochemical cell components. The subassembly is compressed andheated to a temperature that is between about the melting pointtemperature of the bonding element and about the lowest melting pointtemperature of the any one of the two or more electrochemical cellcomponents. Preferably the temperature is less than 800° C., morepreferably below 250° C. The subassembly is then allowed to cool.

The subassembly is preferably positioned into an electrochemical cell oran electrochemical cell stack. The two or more electrochemical cellcomponents are preferably metal components selected from plates, shims,frames, flow fields or combinations thereof, such as stainless steel,titanium, nickel, nickel plated aluminum, nickel plated magnesium, orcombinations thereof. The bonding element is preferably solder. Themetal component is preferably dipped in a flux; then dipped in a bondingmetal or solder, such as tin or a silver-tin alloy. The bonding metal,or solder can also be applied to the metal surface by electrodepositionor by various vacuum deposition techniques.

Light or easily oxidized metal components, such as those made fromaluminum, magnesium, or alloys containing aluminum or magnesium arepreferably coated with a layer of a corrosion resistant transition metalprior to the dipping the metal component in the flux. Suitable corrosionresistant transition metals include but are not limited to cobalt,copper, silver, nickel, gold or combinations thereof. Nickel is the mostcommonly used metal for the corrosion resistant layer.

Alternatively, the two or more electrochemical cell components can bepolymer components selected from frames, gaskets, membranes, shims, orcombinations thereof where the bonding element is preferably anadhesive. The two or more electrochemical cell components may alsocomprise one or more metal components and one or more polymercomponents, where the bonding element is an adhesive.

The two or more electrochemical cell components can include a plate anda flow field. The subassembly preferably includes a bipolar plate and aframe. The bipolar plate preferably has two plates, a flow field and aframe. The frame and flow field are disposed between the two plates withthe frame disposed around the flow field. The frame has channels influid communication with the flow field.

In another embodiment of the invention, there is provided a fluid cooledbipolar plate assembly having two electronically conducting plateshaving opposing faces, an electronically conducting flow field bonded inelectronic communication with a substantial portion of the opposingfaces of the plates, between the two electronically conducting plates,and a frame disposed around a perimeter of the electronically conductingflow field and bonded between the two electronically conducting plates.The frame has channels for providing fluid communication between theflow field and a fluid source.

Preferably, an electronically conducting cathode flow field and anelectronically conducting anode flow field are bonded to opposing sidesof the assembly. The assembly is preferably bonded with an adhesiveand/or solder.

In yet another embodiment there is provided a bipolar plate forelectrochemical cells having two or more porous, electrically conductingsheets selected from expanded metal mesh, woven metal mesh, metal foam,conducting polymer foam, porous conductive carbon material orcombinations thereof. An electrically conducting gas barrier is disposedin electrical contact between the sheets. A cell frame is disposedaround a periphery of any one of the two or more porous electricallyconducting sheets. The cell frame has at least one surface that isbonded to the gas barrier.

Preferably, the cell frame includes channels in fluid communication withthe porous electrically conducting sheet. The cell frame can bemetallic, where it is bonded to the gas barrier with a metallic bond.The metallic bond is preferably formed by soldering the cell frame tothe gas barrier. Alternatively, the cell frame can be polymeric, whereit is bonded to the gas barrier with a polymeric bond. Preferably thepolymeric bond is produced by an adhesive.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the above recited features and advantages the present inventioncan be understood in detail, a more particular description of theinvention, briefly summarized above, may be had by reference to theembodiments thereof which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a face view of a typical metal separator plate with aserpentine flow field design.

FIG. 2 is a schematic cross-section of a PEM fuel cell. The relativethickness of the components has been greatly exaggerated for clarity.

FIG. 3 is a partial cross-section of a bipolar plate having a metal gasbarrier with metal flow fields and polymer cell frames.

FIG. 4 is a schematic drawing of the flat components used in the threedimensional structure of a fluid cooled bipolar plate.

FIG. 5 is a graph showing the performance of a fuel cell stack of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to components for use in electrochemicalcells and methods for fabricating those components, including bipolarseparator plates. More particularly, the present invention relates to amethod for bonding adjacent components of an individual cell and/oradjacent cells of a stack. The bonding provides gas tight seals whichreduces or eliminates the need for certain gasket components and reducesor eliminates certain electronic contact resistances.

One aspect of the invention provides a bipolar plate for anelectrochemical cell having an electrically conducting flow field, atleast one gas barrier and a cell frame. The components of the bipolarplate are bonded to each other to form gas tight bonds with eitherpolymeric type bonds or metallic type bonds.

Another aspect of the present invention provides a method for bondingconductive portions of a bipolar plate. The conductive pieces are tinnedor plated to coat them with a conductive metal and subsequently bondedtogether under pressure and heat to form bonds between the parts.

The component parts of a bipolar plate or other member of anelectrochemical cell may be categorized as being either those that mustbe conductive for the stack to function or those that are not requiredto be conductive. The flow fields and the gas barriers of the bipolarplate must be fabricated from a conductive material, such as a metal ora conductive form of carbon, such as graphite. The cell frames and thesealing surfaces, also known as shims may be electronically insulatingor conductive and can optionally be fabricated from various polymers,thereby resulting in a lower density component and significant overallweight savings for the cell or stack.

When individual components of a stack are fabricated from polymers andother components are fabricated from metals, there are three possibletypes of interfaces present in a fuel cell stack, namelypolymer-to-metal interfaces, polymer-to-polymer interfaces (such as ifthe membrane and electrode assemblies (M&E's) are bonded in place) andmetal-to-metal interfaces. Each of these types of interfaces are bondeddifferently to form an electrochemical cell stack.

FIG. 3 is a partial cross-section of a bipolar plate 48 having a metalgas barrier 50 with metal flow fields 52 and polymer cell frames 54,which essentially take the place of gaskets. The flow fields 52 areshown here as foamed metal, but may be fabricated from expanded metal,perforated metal, metal mesh screens or other porous conductivematerials, as described in U.S. patent application Ser. No. 08/787,271,which is incorporated by reference in its entirety herein. The metal gasbarrier 50 extends beyond the flow fields 52, preferably to the edge ofthe stack, with separate cell frames 54 on both the anode and cathodesides of the gas barrier 50. Flow fields 52 are placed against thebarrier 50, and inside the frames 54 as shown. Additional gasketing maybe included between the barrier and the frame if desired.

FIG. 4 is a schematic drawing of the components used in the threedimensional structure of a fluid cooled bipolar plate. A fluid cooledbipolar plate 55 is shown assembled from a series of substantiallyplanar components including two cooling fluid barriers 60 and a coolingfluid frame 62, with an electronically conducting flow field 63 therein,similar to the flow field 52 of FIG. 3. Optionally, the fluid cooledbipolar plate 55 may further include an anode cell frame 58, cathodecell frame 64 and sealing plates 56 (for contacting and securing a PEMor membrane and electrode (M&E) assembly).

This bipolar plate includes an internal cooling flow field 63 thatallows passage of a cooling fluid for cooling the stack. The coolingfluid flows from a cooling fluid inlet manifold 65, through the coolingflow field 63 within the cooling fluid frame 62, and into a coolingfluid outlet manifold (not shown) generally similar to, but opposedfrom, the inlet manifold 65. The flow field 67 in the anode cell frame58, the flow field 63 in the cooling fluid frame 62 and the flow field69 in the cathode cell frame 64 may be made from any porous conductivematerial as described above.

Bonding Polymers-to-Metals

Bonding the polymer elements of a lightweight PEM fuel cell stack, suchas the anode and cathode cell frames, to metallic elements, such as thegas barriers may be done with an adhesive that bonds to both surfaces toform a gas-tight seal that remains stable under typical cell operatingconditions.

Four types of polymers were tested for fabricating components for PEMfuel cells (polycarbonate, polyethersulfone, polyetherimide, andpolyimide). These polymers are representative of thermoplasticsconsidered to be useful for this application. The polymer sheets werebonded to gold-plated titanium sheets using different adhesives todetermine the combinations that produced adequate bonding for use inelectrochemical cells. Gold plated titanium was chosen for theseexperiments because it is considered to be one of the most challengingmaterials, useful in conductive electrochemical cells components, tobond with a polymer.

A variety of adhesives were used to bond 0.032″ (0.813 mm) polymersheets to gold plated 0.007″ (0.178 mm) titanium sheets. The testspecimens were approximately 2 centimeters wide and 2 centimeters long.The procedure involved preparing the surfaces, applying the adhesive,bonding, and curing the adhesive according to the individualmanufacturers instructions. The bonded specimens were then exposed towater at 60° C. for 24 hours, after which they were examined todetermine the quality of the bond. A representative group of adhesivesshowing reasonable bonding characteristics are included in Table I.

TABLE I Adhesion Testing of Adhesively Bonded Polymer/Metal Couples.Polymer Adhesive Results Polycarbonate Acrylic-epoxy Fair adhesion toboth surfaces hybrid and fair cohesion Polycarbonate Butadiene Goodadhesion to both surfaces; fair cohesion Polycarbonate Acrylic Fairadhesion to both surfaces; poor cohesion Polycarbonate Polyurethane Bondfails at Ti surface when wet Polycarbonate Neoprene Good adhesion andcohesion properties but pigment leaches into water Polycarbonate Acrylicw/rubber Bond fails at Ti surface when wet Polycarbonate Heat SealExcellent adhesion and cohesion properties when wet PolyethersulfoneButadiene Good adhesion to both surfaces; fair cohesion PolyethersulfonePolyurethane Bond fails at Ti surface when wet Polyethersulfone NeopreneExcellent adhesion and cohesion properties but pigment leaches intowater Polyetherimide Butadiene Good adhesion to both surfaces but faircohesion Polyetherimide Polyurethane Bond fails at Ti surface when wetPolyetherimide Neoprene Excellent adhesion and cohesion properties butpigment leaches into water Polyetherimide Acrylic w/rubber Bond fails atgold plated Ti surface when wet Polyetherimide Heat Seal Bond fails atgold plated Ti surface when wet Polyimide Silicone Bond fails at plasticsurface Polyimide Pd/Ag Epoxy Good adhesion and cohesion propertiesPolyimide Acrylic-epoxy Bond fails at plastic surface hybrid

TABLE II Product and Manufacturer Identification for the AdhesivesAppearing in Table I Adhesive Product Name Manufacturer Acrylic- PLASTICDevcon Inc., Danvers, MA epoxy hybrid WELDER II Butadiene GOOP and E6000Eclectic Products, Carson, CA STIK ‘N SEAL Loctite, Inc., Newington, CNAcrylic Acrylic 204 and Lord Adhesives, Erie, PA Acrylic 406Polyurethane PLIOGRIP 7775L Ashland Chemical, Columbus, OH NeopreneSCOTCH GRIP 1357 3M, St Paul, MN Acrylic Acrylic 330 Loctite, Inc.,Newington, CN w/rubber Heat Seal Polyurethane 3218 Bemis Assoc. Inc.,Shirley, MA Silicone 748 Silicone Rubber Dow-Corning, Midland, MI CementPd/Ag Pd/Ag filled Epoxy Epotek, Inc., Billerica, MA Epoxy

This list is considered to be representative and in no way exhaustive ofthe types of materials useful in carrying out the present invention.

Bonding Metals-to-Metals

Two types of metal-to-metal bonds are present in an electrochemicalcell. There are bonds that are critical to the conductive path ofelectrons, such as the bonding of flow fields to gas barriers, and thereare bonds that are critical to seal against fluid leakage, such as thebonding of gas barriers to metal frames. With regard to the gas tightbonds, it is beneficial, but typically not necessary, if the bond isalso conductive.

Conductive bonds can be created between a variety of metals by solderingthe parts together. Soldering is a well known technique where arelatively low melting metal is used to bond two components togetherthat are fabricated from metals having higher melting points than thelow melting metal or solder. It is imperative that the low melting metalwets the higher melting metals to achieve a good bond.

Conventional soldering for wire attachment for an electrical assemblyinvolves heating the parts to above the melting point of the solder,then applying a flux to remove the oxide film on the metal and finallyapplying the solder, with the flux and the solder sometimes appliedtogether. Soldering large areas requires a different process. In orderto solder large flat areas, the part is first coated with thebond-forming metal or alloy by one of several methods. It can first becoated with flux and then coated with solder in a process known astinning. The flux can be applied in a variety of ways, however brushingand dipping are the most commonly used methods. Application of solder istypically accomplished by dipping the part in a container of moltensolder. Specific approaches to tinning parts fabricated from stainlesssteel, nickel, titanium, magnesium, aluminum, and alloys containingaluminum and magnesium are described below. The bond forming metal oralloy can also be plated onto the metal surface by eitherelectrodeposition or electroless deposition, both of which are wellknown processes. Also, the bond forming metal or alloy can be depositedonto the metal surface using vacuum deposition techniques, such asevaporation, chemical vapor desposition and sputtering.

After depositing the bond forming metal by tinning or plating, the partsare bonded by clamping the flat surfaces to be bonded together andreheating the assembly to above the softening point of the solder. Boththe clamping and the heating can be accomplished simultaneously throughthe use of a hot press.

The procedure for tinning stainless steel parts includes, dipping thepart in an acid flux solution; slowly dipping the part in molten solder;dipping the part in the acid flux a second time; and dipping the part inmolten solder a second time. Preferably, the tinning procedure iscarried out in an inert atmosphere. The clean stainless steel part isimmersed in a water soluble flux, preferably an acid flux, and theexcess solution is allowed to drain off. The part is then slowly loweredinto the molten solder. It is important that the part be immersedslowly, because the flux is only active for removing the oxide filmwhile the flux is hot, and a slow immersion process leads to a longerexposure time and better oxide removal. A silver solder, such aseutectic tin-silver (96.5 Sn:3.5 Ag) is preferred. In some cases, asingle immersion in the molten solder is sufficient, but in other casesa second dipping is preferred. In the latter case, after the part isremoved from the solder, it is allowed to cool slightly to insure thatthe solder has solidified, and immersed in the flux a second time. Thepart is then removed from the flux and the excess solution allowed todrain off. Ideally, the part should be sufficiently hot to boil the fluxas it makes contact. In the last step, the part is slowly lowered intothe molten solder and removed.

The first immersion in the solder covers most of the surface of thestainless steel part with a thin layer of solder, however occasionallythere are areas where the flux did not remove all of the oxide film,that are poorly coated. The second dip in the flux with the part hotremoves any remaining oxide film from the part and produces a shiny,mirror-like finish on the metal. It is important that all grease andsuperficial dirt be removed from the surface of the metal beforesoldering. Failure to clean and degrease the surface may leave aprotective film on the oxide layer that prevents the flux from cleaningthe surface, and prevents the solder from sticking. After tinning, theparts are preferably thoroughly washed in deionized water to remove allremaining traces of the flux. Because water soluble flux residues floaton the surface of the molten solder, most parts will have traces of fluxpresent on the tinned surface. Other types of flux may be used to carryout the tinning process.

The same tinning procedure works for titanium, aluminum, magnesium, andalloys containing these elements as well, if they are first plated withnickel, copper, or another metal that more readily accepts tinning.Precious or base metals may be used to plate the titanium.

The procedure for tinning titanium, aluminum, and magnesium containingparts includes plating the part with nickel, dipping the part in an acidflux solution, and dipping the part in molten solder. In order to nickelplate the part, the oxide film must be removed.

After the part has been nickel plated, it is coated with flux and dippedin molten solder to produce a shiny tinned finish using the same cycleand silver solder as is used with stainless steel. Generally, only asingle immersion is required for each the flux and the solder withnickel-plated aluminum.

Both aluminum and titanium can be tinned directly if the propercombination of flux and solder are employed. With the use of a highviscosity liquid flux, aluminum and titanium can be directly tinned. Inthis process the part is first immersed in the flux, and then slowlyimmersed in molten solder, with a single cycle being sufficient. Forthese metals, a solder with a more active component is preferred, suchas eutectic tin-zinc (91% Sn, 9% Zn, melting point 199° C.).

It is important to note that for best results, all of the tinningoperations described herein are preferably carried out in an oxygen freeatmosphere, or an atmosphere with a substantially reduced oxygencontent. It is not necessary, and indeed in most cases it is notpossible, to exclude water, since water is a major component of manyfluxes.

As long as a tin based solder is being used, it doesn't matter whatalloying elements are present in the solder when forming the finalbonds, the tinned parts can readily be bonded together. When solderswith dissimilar compositions are used to tin metals to be bonded it isimportant to heat the clamped components to at least the melting pointof the higher melting solder.

While the metallurgical bond produced by soldering metal componentstogether is preferred, it is not the only approach to joining metalcomponents. Adhesives, like those listed in Table II, can be used forsome metal-to-metal bonds as well. In general, these adhesives aresuitable for bonds where through-bond electrical conductivity is notrequired. Except for the Pd—Ag filled epoxy, none of the adhesiveslisted in Table II are conducting. The conductive adhesive bond may besubstituted for a soldered joint in producing an electrochemical cell orstack in accordance with the present invention.

Still other metal joining processes applicable to this type of stackfabrication will be apparent to those skilled in the art of metaljoining. These are also considered to be part of this invention whenthey are employed for the assembly of an electrochemical cell stack in amanner essentially similar to that described here.

Bonding Polymers to Polymers

There are many adhesives available which are suitable for bondingengineering thermoplastics to each other. However, bonding PEM membranesto other materials is more difficult because perfluorinated membranesare resistant to adhesion by most adhesives. Dow-Coming 748™ adhesive(see line 8 in Table II) adheres well to the membrane and to many metalsurfaces. While the peel strength, at about 1.3 kg per centimeter ofbond, is inadequate for most structural purposes, it is more thanadequate to produce a thin, fully adherent seal for use in anelectrochemical cell.

Assembly Procedures

Assembling a fully bonded electrochemical cell stack includes severalsteps. First, all of the metal parts are coated with the bonding metal,including the anode flow field, cathode flow field, and cooling flowfield; the gas barriers; and the frame around the cooling flow field.These parts are then stacked into position, carefully positioned, andheld in place using an alignment device or jig. The parts and the jigare then placed in a hot press to clamp the parts together, pressing thegas barriers firmly against the cooling frame and the flow fields firmlyagainst the gas barriers. The press is heated to slightly above themelting point of the highest melting bonding metal present in order toremelt the bonding metal. The bonded assembly is then allowed to cool inthe press.

Sheets of hot melt adhesive are cut to match the size and shape of theanode and cathode cell frames. These are used to bond the cell frames tothe cell shims through the use of the hot press. Those two piecesubassemblies are bonded to the assembled bipolar plate by the sametechnique. All of the hot melt bonds can be made in a single step, ifdesired.

An uncooled bipolar plate is somewhat easier to fabricate because itcontains few parts. The flow fields are bonded to opposite sides of agas barrier by soldering. The anode and cathode cell frames are attachedto the gas barriers and the shims with hot melt adhesives in one or twopressing operations.

The following examples show some of the preferred embodiments of thepresent invention, however they are not to be considered limiting in anysense.

EXAMPLE 1

This example shows the fabrication of the core of a liquid cooledbipolar plate.

Two titanium gas barriers were fabricated from 0.0045″ (0.114 mm) metalsheet and plated with gold. A frame was fabricated from 0.032″ (0.813mm) polyether-sulfone sheet. A flow field was fabricated from threelayers of expanded titanium, one with a thickness of about 0.030″ (0.762mm) at full expansion and two which were expanded and subsequentlyflattened back to their original 0.003″ (0.076 mm) thickness. All threesheets were spot welded together, with the 0.003″ sheets welded toopposing faces of the 0.030″ sheet, and the welded assembly was goldplated. This flow field was deliberately produced thicker than the frameto insure that the flow field, which compresses like a spring, would putpressure on the gas barriers to ensure electrical contact with thebarriers.

The faces of the polyethersulfone frame were coated with ECLECTIC E6000adhesive. The frame was then pressed, by hand, against one gas barrier,with both parts kept in alignment by the use of a positioning jigfabricated for this purpose and equipped with alignment pins to insurethat all of the parts stay precisely in alignment. The flow field wasplaced in the cavity produced by the frame on the barrier, and thesecond gas barrier was pressed on top of the other parts. The assembly,still in the assembly jig, was placed in a hot press. The componentswere then pressed together and cured under sufficient compression tokeep the parts from moving at 60° C. for 24 hours. The result was abipolar plate having a gas barrier with fluid access to the internalflow field suitable for passage of a cooling fluid for the purpose ofremoving waste heat from the fuel cell stack. In subsequent assemblysteps, this subassembly was treated in the same manner as a simple gasbarrier sheet.

EXAMPLE 2

This example shows the bonding of two components with hot melt adhesive.

A face shim or sealing plate 56 was fabricated from 0.0045″ (0.114 mm)titanium sheet, as illustrated in FIG. 4. An anode cell frame wasfabricated from 0.032″ (0.813 mm) polyethersulfone sheet. The anode cellframe was used as a template to cut a sheet of hot melt adhesive (BEMISnumber 3218). All three pieces were stacked in a positioning jigfabricated for this purpose and equipped with alignment pins to insurethat all of the parts stay precisely in alignment with the adhesivebetween the metal shim and the polymer frame. The jig, with the partsinside of it, was placed in a hot press, compressed to insure andmaintain intimate contact between the sheet components, and heated to150° C. (The temperature used for this step is a function of theadhesive used.) After several minutes, the press was cooled, and thesubassembly removed.

This subassembly is suitable for use as the frame around the anode flowfield and the contact surface against the membrane portion of a membraneand electrode assembly (M&E) and used in assembling a fuel stack.

EXAMPLE 3

This example shows the tinning of stainless steel. A gas barrier wasfabricated from 0.010″ (0.254 mm) stainless steel sheet. The gas barrierand the other items required to carry out the tinning operation (flux,solder, heater, tongs, temperature indicator, etc.) are placed in aglove box and the atmosphere of the box thoroughly purged with argon toprevent oxidation of freshly cleaned surfaces. The barrier is immersedin an acid type liquid flux such as LA-CO N-3 (LA-CO Industries,Chicago, Ill.). Excess liquid is allowed to drain-off stainless steelsheet, and the piece is slowly immersed in molten silver-tin eutecticsolder (3.5% Ag, 96.5% Sn, melting point 221° C., (Kester Solder, DesPlaines, Ill.). It is important to immerse the piece slowly, since theflux is only active and capable of dissolving the surface oxide layer onthe metal over a limited temperature range, and a slow immersion insuresan adequate exposure of the entire surface of the part to the flux atits active temperature, but before all of the salts active fordissolving the oxide film vaporize.

After immersion, the part is removed from the molten solder, with allparts of the surface which were stripped of oxide by the action of theflux being coated with solder. The part is allowed to cool until thesolder solidifies before being re-immersed in the flux. It is preferableto have the part remain sufficiently hot to boil the liquid slightly asit contacts the surface of the metal. The part is removed from the fluxand slowly re-immersed in the solder, with the second cycle serving toremove the oxide film from the parts of the surface which were notstripped the first time and coat those areas with solder. After the partis removed from the molten solder it is allowed to cool in the air untilthe solder solidifies. The part may then be laid down on a heatresistant surface or other support to finish cooling. The result is amirror-like finish on the tinned part.

EXAMPLE 4

This example shows the tinning of aluminum. A cooling cell frame for afluid cooled bipolar separator plate was fabricated from 0.020″ (0.508mm) aluminum sheet and plated with nickel. The gas barrier and the otheritems required to carry out the tinning operation (flux, solder, heater,tongs, temperature indicator, etc.) were placed in a glove box and theatmosphere of the box was thoroughly purged with argon to preventoxidation of freshly cleaned surfaces. The barrier was immersed in anacid type liquid flux such as LA-CO N-3 (LA-CO Industries, Chicago,Ill.). Excess liquid was allowed to drain-off, and the piece was slowlyimmersed in molten silver-tin eutectic solder (3.5% Ag, 96.5% Sn) havinga melting point of 221° C. (Kester Solder, Des Plaines, Ill.). It isimportant to immerse the piece slowly, since the flux is only active andcapable of dissolving the surface oxide layer on the metal over alimited temperature range, and immersing the part slowly ensures thatthe entire surface of the part is adequately exposed to the flux at itsactive temperature, but before the volatile components are vaporized.After immersion, the part is removed from the molten solder with thesurface completely coated with solder.

EXAMPLE 5

This example shows the bonding of aluminum and stainless steelcomponents to form the core of a liquid cooled bipolar plate usingsolder.

Two gas barriers and a flow field are tinned as described in Example 3.An aluminum cooling frame is tinned as described in Example 4. Thecomponents are stacked as shown in FIG. 4 and placed in a hot press witha thermocouple in contact with the side of the frame and clamped firmlytogether. The press is heated until the thermocouple indicates that theload has reached and maintained a temperature of about 230° C. for aboutfive minutes, at which time the power to the heaters is turned off andthe entire assembly is allowed to cool while maintaining the assembly ina compressed state. The result is a water cooled gas barrier with fluidaccess to the internal flow field suitable for use as a component in afuel cell stack with the specific purpose of removing waste heat fromthe stack. This subassembly was treated in the same manner as one of thesimple gas barrier sheets in subsequent assembly steps.

EXAMPLE 6

This example illustrates the performance of a fuel cell stack producedin accordance with the present invention.

The method of Example 1 was used to prepare a set of four water cooledbipolar plate cores sized for use in a fuel cell stack with an activearea for 125 cm². Three uncooled bipolar plates were also used in thestack. The method of Example 2 was used to prepare eight anode cellframes with shims and eight cathode cell frames with shims for a stackof that size. Additional components (M&E's, flow fields, gaskets, etc.,)were also prepared by conventional means. These were used to fabricatean eight cell PEM fuel cell stack. The stack was operated using hydrogenfuel with air as the oxidant. FIG. 5 illustrates the performance of thateight-cell stack, 125 cm² per cell active area while operating at an M&Etemperature of 62-64° C., a fuel gas pressure of 15 psig, both fuel andoxidant gases humidified to a dew point of 27° C., and air supplied atfour-fold stoichiometry. The stack provided an output of 967 W/kg and846 W/L for the repeating units of this stack which was determined to bean efficiency of 53.4% (at a potential of 0.651 V/cell).

While the foregoing is directed to preferred embodiments of the presentinvention, other and further embodiments of the invention may be devisedwithout departing from the basic scope thereof, and the scope thereof isdetermined by the claims which follow.

What is claimed is:
 1. A fluid cooled bipolar plate assembly comprising:two electronically conducting fluid impermeable plates; anelectronically conducting cooling fluid permeable flow field bonded inelectronic communication between the two electronically conductingplates; and a frame disposed around a perimeter of the cooling fluidpermeable flow field and bonded between the two electronicallyconducting plates, wherein the frame has channels for providing fluidcommunication between the cooling fluid permeable flow field and a fluidsource.
 2. The assembly of claim 1, further comprising an electronicallyconducting cathode flow field and an electronically conducting anodeflow field bonded to opposing sides of the assembly.
 3. The assembly ofclaim 1, wherein the assembly is bonded with an adhesive.
 4. Theassembly of claim 1, wherein the assembly is bonded with solder.
 5. Theassembly of claim 1, wherein the two fluid impermeable plates, the flowfield, and the frame are made from stainless steel, titanium, aluminum,magnesium, nickel, nickel-plated aluminum, nickel-plated magnesium,copper-plated aluminum, copper-plated magnesium, gold-plated titanium orcombinations thereof.
 6. The assembly of claim 1, wherein the frame isselected from polycarbonate, polyethersulfone, polyetherimide,polyimide, or combinations thereof.
 7. The assembly of claim 4, whereinthe solder is selected from tin, silver, zinc, bismuth, lead, indium,and alloys thereof.
 8. The assembly of claim 6, wherein the frame isadhesively bonded to the two fluid impermeable plates.
 9. The assemblyof claim 1, wherein the two fluid impermeable plates form gas barrierson opposing sides of the flow field.
 10. The assembly of claim 1,wherein the two plates and the flow field are electronically conductingindividually and as a unitary bonded assembly.
 11. The assembly of claim10, wherein the electronically conducting plates and flow field aremetals.
 12. The assembly of claim 1, further comprising anelectronically conducting cathode flow field and an electronicallyconducting anode flow field bonded to opposing sides of the assembly.13. The assembly of claim 12, wherein the cathode flow field and theanode flow field are metal.
 14. The assembly of claim 12, wherein thetwo plates, the flow field, the cathode flow field, and the anode flowfield are electronically conducting individually and as a unitary bondedassembly.
 15. The assembly of claim 12, wherein the fluid permeable flowfield, the cathode flow field, and the anode flow field are porouselectrically conducting sheets selected from expanded metal mesh, wovenmetal mesh, metal foam, perforated metal sheet, conducting polymer foam,porous conductive carbon material, or combinations thereof.
 16. Theassembly of claim 12, further comprising a cathode cell frame disposedaround a perimeter of the cathode flow field, an anode cell framedisposed around a perimeter of the anode flow field and wherein thecathode and anode cell frames have channels in fluid communicationbetween the cathode flow field and the anode flow field, respectively,and fluid sources.
 17. The assembly of claim 1, wherein the two fluidimpermeable plates, the flow field, and the frame are bonded to eachother to form gas tight bonds with either polymeric bonds or metallicbonds.
 18. The assembly of claim 15, wherein the polymeric bonds ormetallic bonds are stable under electrochemical cell stack operatingconditions.
 19. The assembly of claim 6, wherein the assembly has atleast one interface selected from a polymer-to-metal interface,polymer-to-polymer interface, and metal-to-metal interface.
 20. Theassembly of claim 1, wherein the flow field allows passage of a coolingfluid between the two plates for cooling the electrochemical cell stack.21. The assembly of claim 1, wherein the fluid cooled bipolar plate iscapable of supporting current densities of about 0.8 Amps per squarecentimeter.
 22. The assembly of claim 19, wherein the two electronicallyconducting plates, the electronically conducting fluid permeable flowfield, and the frame are fabricated using a process selected frommolding, casting, and stamping.
 23. The assembly of claim 22, furthercomprising a cathode cell frame disposed around a perimeter of thecathode flow field, an anode cell frame disposed around a perimeter ofthe anode flow field, and wherein the cathode and anode cell frames havechannels in fluid communication between the cathode flow field and anodeflow field, respectively, and fluid sources.
 24. A bipolar plate forelectrochemical cells, comprising: two or more porous, electricallyconducting sheets selected from expanded metal mesh, woven metal mesh,metal foam, conducting polymer foam, porous conductive carbon materialor combinations thereof; an electrically conducting gas barrier disposedin electrical contact between the sheets; and a cell frame disposedaround a periphery of any one of the two or more porous electricallyconducting sheets, wherein the cell frame has at least one surface thatis bonded to the gas barrier.
 25. The bipolar plate of claim 24, whereinthe cell frame includes channels in fluid communication between one ofthe two or more porous electrically conducting sheets and a fluidsource.
 26. The bipolar plate of claim 25, wherein the cell frame ismetallic and is bonded to the gas barrier with a metallic bond.
 27. Thebipolar plate of claim 25, wherein the cell frame is polymeric and isbonded to the gas barrier with a polymeric bond.
 28. The bipolar plateof claim 26, wherein the metallic bond is formed by soldering the cellframe to the gas barrier.
 29. The bipolar plate of claim 27, wherein thepolymeric bond is produced by an adhesive.
 30. The bipolar plate ofclaim 24, wherein the metal sheet gas barrier and the two porous metalsheets are metallurgically bonded over an entire active area of theelectrochemical cells.
 31. The bipolar plate of claim 24, wherein themetal sheet gas barrier and the two porous metal sheets aremetallurgically bonded by solder.
 32. A bipolar plate forelectrochemical cells, comprising: two porous metal sheet flowfields; ametal sheet gas barrier metallurgically bonded between the two porousmetal sheet flowfields; and a cell frame disposed around a periphery ofone or more of the two porous metal sheet flowfields, wherein the cellframe has at least one surface that is bonded to the metal sheet gasbarrier.
 33. The bipolar plate of claim 32, wherein the metal sheet gasbarrier and the two porous metal sheets are metallurgically bonded overan entire active area of the electrochemical cells.
 34. The bipolarplate of claim 32, wherein the two porous metal sheet flowfields arecontinuous over the entire active area of the electrochemical cells. 35.The bipolar plate of claim 32, wherein the metal sheet gas barrier andthe two porous metal sheets are metallurgically bonded by solder. 36.The bipolar plate of claim 32, wherein the metal sheet gas barrier andthe two porous metal sheets are metallurgically bonded alongmetal-to-metal interfaces therebetween.
 37. The assembly of claim 1,wherein the two fluid impermeable plates and the cooling fluid permeableflow field are metal and are metallurgically bonded together by solder.38. The assembly of claim 1, wherein the two fluid impermeable platesand the cooling fluid permeable flow field are metal and aremetallurgically bonded together along metal-to-metal interfacestherebetween.
 39. The bipolar plate of claim 32, wherein each cell frameincludes channels in fluid communication between one of the two porousmetal sheet flow fields and a fluid source.